When paramecia are placed in homologous antisera, they become immobilized and eventually die. This specific reaction has been shown to be due to a series of proteins associated with the cell surface. The antigen is distributed over 90 percent of the cell membrane and exists in a homogeneous state. While a clonal population of cells expresses only one antigen (serotype) at any given time, individual cells are capable of expressing upwards of twelve different antigens. This antigenic variation has been shown to exist now in a wide variety of species of the protozoa including both free-living forms (Paramecium) and parasitic forms (Trypanosomes). The mechanisms of antigenic variation are still unknown. An ultimate objective of this project is the partial sequence comparisons between the various antigenic types both within a single species and between several related species.